Internet Engineering Task Force (IETF)                    P. Saint-Andre
Request for Comments: 7565                                      May 2015
Category: Standards Track
ISSN: 2070-1721


                         The 'acct' URI Scheme

Abstract

   This document defines the 'acct' Uniform Resource Identifier (URI)
   scheme as a way to identify a user's account at a service provider,
   irrespective of the particular protocols that can be used to interact
   with the account.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7565.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................2
   3. Rationale .......................................................2
   4. Definition ......................................................3
   5. Security Considerations .........................................4
   6. Internationalization Considerations .............................5
   7. IANA Considerations .............................................5
   8. References ......................................................6
      8.1. Normative References .......................................6
      8.2. Informative References .....................................7
   Acknowledgements ...................................................8
   Author's Address ...................................................8

1.  Introduction

   Existing Uniform Resource Identifier (URI) schemes that enable
   interaction with, or that identify resources associated with, a
   user's account at a service provider are tied to particular services
   or application protocols.  Two examples are the 'mailto' scheme
   (which enables interaction with a user's email account) and the
   'http' scheme (which enables retrieval of web files controlled by a
   user or interaction with interfaces providing information about a
   user).  However, there exists no URI scheme that generically
   identifies a user's account at a service provider without specifying
   a particular protocol to use when interacting with the account.  This
   specification fills that gap.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

3.  Rationale

   During formalization of the WebFinger protocol [RFC7033], much
   discussion occurred regarding the appropriate URI scheme to include
   when specifying a user's account as a web link [RFC5988].  Although
   both the 'mailto' [RFC6068] and 'http' [RFC7230] schemes were
   proposed, not all service providers offer email services or web
   interfaces on behalf of user accounts (e.g., a microblogging or
   instant messaging provider might not offer email services, or an
   enterprise might not offer HTTP interfaces to information about its
   employees).  Therefore, the participants in the discussion recognized
   that it would be helpful to define a URI scheme that could be used to

   generically identify a user's account at a service provider,
   irrespective of the particular application protocols used to interact
   with the account.  The result was the 'acct' URI scheme defined in
   this document.

   (Note that a user is not necessarily a human; it could be an
   automated application such as a bot, a role-based alias, etc.
   However, an 'acct' URI is always used to identify something that has
   an account at a service, not the service itself.)

4.  Definition

   The syntax of the 'acct' URI scheme is defined under Section 7 of
   this document.  Although 'acct' URIs take the form "user@host", the
   scheme is designed for the purpose of identification instead of
   interaction (regarding this distinction, see Section 1.2.2 of
   [RFC3986]).  The "Internet resource" identified by an 'acct' URI is a
   user's account hosted at a service provider, where the service
   provider is typically associated with a DNS domain name.  Thus, a
   particular 'acct' URI is formed by setting the "user" portion to the
   user's account name at the service provider and by setting the "host"
   portion to the DNS domain name of the service provider.

   Consider the case of a user with an account name of "foobar" on a
   microblogging service "status.example.net".  It is taken as
   convention that the string "foobar@status.example.net" designates
   that account.  This is expressed as a URI using the 'acct' scheme as
   "acct:foobar@status.example.net".

   A common scenario is for a user to register with a service provider
   using an identifier (such as an email address) that is associated
   with some other service provider.  For example, a user with the email
   address "juliet@capulet.example" might register with a commerce
   website whose domain name is "shoppingsite.example".  In order to use
   her email address as the localpart of the 'acct' URI, the at-sign
   character (U+0040) needs to be percent-encoded as described in
   [RFC3986].  Thus, the resulting 'acct' URI would be
   "acct:juliet%40capulet.example@shoppingsite.example".

   It is not assumed that an entity will necessarily be able to interact
   with a user's account using any particular application protocol, such
   as email; to enable such interaction, an entity would need to use the
   appropriate URI scheme for such a protocol, such as the 'mailto'
   scheme.  While it might be true that the 'acct' URI minus the scheme
   name (e.g., "user@example.com" derived from "acct:user@example.com")
   can be reached via email or some other application protocol, that
   fact would be purely contingent and dependent upon the deployment
   practices of the provider.

   Because an 'acct' URI enables abstract identification only and not
   interaction, this specification provides no method for dereferencing
   an 'acct' URI on its own, e.g., as the value of the 'href' attribute
   of an HTML anchor element.  For example, there is no behavior
   specified in this document for an 'acct' URI used as follows:

   <a href='acct:bob@example.com'>find out more</a>

   Any protocol that uses 'acct' URIs is responsible for specifying how
   an 'acct' URI is employed in the context of that protocol (in
   particular, how it is dereferenced or resolved; see [RFC3986]).  As a
   concrete example, an "Account Information" application of the
   WebFinger protocol [RFC7033] might take an 'acct' URI, resolve the
   host portion to find a WebFinger server, and then pass the 'acct' URI
   as a parameter in a WebFinger HTTP request for metadata (i.e., web
   links [RFC5988]) about the resource.  For example:

   GET /.well-known/webfinger?resource=acct%3Abob%40example.com HTTP/1.1

   The service retrieves the metadata associated with the account
   identified by that URI and then provides that metadata to the
   requesting entity in an HTTP response.

   If an application needs to compare two 'acct' URIs (e.g., for
   purposes of authentication and authorization), it MUST do so using
   case normalization and percent-encoding normalization as specified in
   Sections 6.2.2.1 and 6.2.2.2 of [RFC3986].

5.  Security Considerations

   Because the 'acct' URI scheme does not directly enable interaction
   with a user's account at a service provider, direct security concerns
   are minimized.

   However, an 'acct' URI does provide proof of existence of the
   account; this implies that harvesting published 'acct' URIs could
   prove useful to spammers and similar attackers -- for example, if
   they can use an 'acct' URI to leverage more information about the
   account (e.g., via WebFinger) or if they can interact with protocol-
   specific URIs (such as 'mailto' URIs) whose user@host portion is the
   same as that of the 'acct' URI.

   In addition, protocols that make use of 'acct' URIs are responsible
   for defining security considerations related to such usage, e.g., the
   risks involved in dereferencing an 'acct' URI, the authentication and
   authorization methods that could be used to control access to
   personal data associated with a user's account at a service, and
   methods for ensuring the confidentiality of such information.

   The use of percent-encoding allows a wider range of characters in
   account names but introduces some additional risks.  Implementers are
   advised to disallow percent-encoded characters or sequences that
   would (1) result in space, null, control, or other characters that
   are otherwise forbidden, (2) allow unauthorized access to private
   data, or (3) lead to other security vulnerabilities.

6.  Internationalization Considerations

   As specified in [RFC3986], the 'acct' URI scheme allows any character
   from the Unicode repertoire [Unicode] encoded as UTF-8 [RFC3629] and
   then percent-encoded into valid ASCII [RFC20].  Before applying any
   percent-encoding, an application MUST ensure the following about the
   string that is used as input to the URI-construction process:

   o  The userpart consists only of Unicode code points that conform to
      the PRECIS IdentifierClass specified in [RFC7564].

   o  The host consists only of Unicode code points that conform to the
      rules specified in [RFC5892].

   o  Internationalized domain name (IDN) labels are encoded as A-labels
      [RFC5890].

7.  IANA Considerations

   In accordance with the guidelines and registration procedures for new
   URI schemes [RFC4395], this section provides the information needed
   to register the 'acct' URI scheme.

   URI Scheme Name:  acct

   Status:  permanent

   URI Scheme Syntax:  The 'acct' URI syntax is defined here in
      Augmented Backus-Naur Form (ABNF) [RFC5234], borrowing the 'host',
      'pct-encoded', 'sub-delims', and 'unreserved' rules from
      [RFC3986]:

      acctURI      = "acct" ":" userpart "@" host
      userpart     = unreserved / sub-delims
                     0*( unreserved / pct-encoded / sub-delims )

      Note that additional rules regarding the strings that are used as
      input to construction of 'acct' URIs further limit the characters
      that can be percent-encoded; see the Encoding Considerations as
      well as Section 6 of this document.

   URI Scheme Semantics:  The 'acct' URI scheme identifies accounts
      hosted at service providers.  It is used only for identification,
      not interaction.  A protocol that employs the 'acct' URI scheme is
      responsible for specifying how an 'acct' URI is dereferenced in
      the context of that protocol.  There is no media type associated
      with the 'acct' URI scheme.

   Encoding Considerations:  See Section 6 of this document.

   Applications/Protocols That Use This URI Scheme Name:  At the time of
      this writing, only the WebFinger protocol uses the 'acct' URI
      scheme.  However, use is not restricted to the WebFinger protocol,
      and the scheme might be considered for use in other protocols.

   Interoperability Considerations:  There are no known interoperability
      concerns related to use of the 'acct' URI scheme.

   Security Considerations:  See Section 5 of this document.

   Contact:  Peter Saint-Andre, peter@andyet.com

   Author/Change Controller:  This scheme is registered under the IETF
      tree.  As such, the IETF maintains change control.

   References:  None.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of
              ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629,
              November 2003, <http://www.rfc-editor.org/info/rfc3629>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <http://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <http://www.rfc-editor.org/info/rfc5234>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <http://www.rfc-editor.org/info/rfc5890>.

   [RFC5892]  Faltstrom, P., Ed., "The Unicode Code Points and
              Internationalized Domain Names for Applications (IDNA)",
              RFC 5892, DOI 10.17487/RFC5892, August 2010,
              <http://www.rfc-editor.org/info/rfc5892>.

   [RFC7564]  Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
              Preparation, Enforcement, and Comparison of
              Internationalized Strings in Application Protocols",
              RFC 7564, DOI 10.17487/RFC7564, May 2015,
              <http://www.rfc-editor.org/info/rfc7564>.

   [Unicode]  The Unicode Consortium, "The Unicode Standard",
              <http://www.unicode.org/versions/latest/>.

8.2.  Informative References

   [RFC20]    Cerf, V., "ASCII format for network interchange", STD 80,
              RFC 20, DOI 10.17487/RFC0020, October 1969,
              <http://www.rfc-editor.org/info/rfc20>.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35,
              RFC 4395, DOI 10.17487/RFC4395, February 2006,
              <http://www.rfc-editor.org/info/rfc4395>.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010,
              <http://www.rfc-editor.org/info/rfc5988>.

   [RFC6068]  Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto'
              URI Scheme", RFC 6068, DOI 10.17487/RFC6068, October 2010,
              <http://www.rfc-editor.org/info/rfc6068>.

   [RFC7033]  Jones, P., Salgueiro, G., Jones, M., and J. Smarr,
              "WebFinger", RFC 7033, DOI 10.17487/RFC7033,
              September 2013, <http://www.rfc-editor.org/info/rfc7033>.

   [RFC7230]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

Acknowledgements

   The 'acct' URI scheme was originally proposed during work on the
   WebFinger protocol; special thanks are due to Blaine Cook, Brad
   Fitzpatrick, and Eran Hammer-Lahav for their early work on the
   concept (which in turn was partially inspired by work on Extensible
   Resource Identifiers at OASIS).  The scheme was first formally
   specified in [RFC7033]; the authors of that specification (Paul
   Jones, Gonzalo Salgueiro, and Joseph Smarr) are gratefully
   acknowledged.  Thanks are also due to Stephane Bortzmeyer, Melvin
   Carvalho, Martin Duerst, Graham Klyne, Barry Leiba, Subramanian
   Moonesamy, Evan Prodromou, James Snell, and various participants in
   the IETF APPSAWG for their feedback.  Meral Shirazipour completed a
   Gen-ART review.  Dave Cridland completed an AppsDir review and is
   gratefully acknowledged for providing proposed text that was
   incorporated into Sections 3 and 5.  IESG comments from Richard
   Barnes, Adrian Farrel, Stephen Farrell, Barry Leiba, Pete Resnick,
   and Sean Turner also led to improvements in the specification.

Author's Address

   Peter Saint-Andre

   EMail: peter@andyet.com
   URI:   https://andyet.com/